With recent increase in the number of crimes such as car thefts, thefts from cars and the like, it has been widely popularized to install a burglar alarm device in a vehicle to sound an alarm by detecting abnormal conditions of the vehicle (e.g., the invasion of criminals on vehicles or the breakage of car windows) (see, e.g., Japanese Patent Application Publication No. 2005-215544).
A burglar, however, may turn off a vehicle burglar alarm device by separating a main battery from the alarm device. For that reason, there has been disclosed a vehicle burglar alarm device including a vehicle burglar alarm circuit capable of sounding an alarm by using an internal battery provided inside the alarm device, even when the main battery is separated from the alarm device.
FIG. 6 is a block diagram showing a conventional vehicle burglar alarm circuit provided in a vehicle burglar alarm device.
The conventional vehicle burglar alarm circuit includes a main battery 11, an internal battery 12, a DC/DC converter 13, a drive circuit unit 14 and a sounding body 15.
The main battery 11 includes a lead rechargeable battery for use in a vehicle. An output voltage V11 of the main battery 11 is a DC voltage of 12 V in the conventional vehicle burglar alarm circuit. The internal battery 12 includes a Ni-MH battery, a lithium battery or the like. An output voltage V12 of the internal battery 12 is a DC voltage ranging from 4.8 to 6.0 V in the conventional vehicle burglar alarm circuit. The DC/DC converter 13 employs as an input power source the main battery 11 when the main battery 11 is mounted on the alarm device and the internal battery 12 when the main battery 11 is separated therefrom. Further, there is provided a switch SW including a semiconductor device such as a field-effect transistor (FET), a transistor or the like for turning on and off a power supplied from the main battery 11 to the DC/DC converter 13.
The DC/DC converter 13 is of a flyback type and configured such that a boosting (step-up) ratio of an output voltage to an input voltage is variable. Further, the DC/DC converter 13 boosts the output voltage V11 and the output voltage V12 having different voltages at different ratios to output a constant output voltage V13. In the conventional vehicle burglar alarm circuit, the output voltage V13 of the DC/DC converter 13 is a DC voltage of 40 V.
The drive circuit 14 includes four transistors (not shown). Further, the drive circuit 14 generates a high frequency output voltage V14 from the output voltage V13 of a DC voltage by turning on and off the four transistors. In this conventional vehicle burglar alarm circuit, the output voltage V14 from the drive circuit 14 is a high frequency voltage of 80 Vpp.
The sounding body 15 is a siren including a ceramic oscillator. When the output voltage V14 is applied to the sounding body 15, the sounding body 15 produces an alarm sound.
With the above configuration, the conventional vehicle burglar alarm drive circuit can produce an alarm sound having the same sound pressure from the sounding body 15 regardless whether it is powered from the main battery 11 or the internal battery 12.
The conventional vehicle burglar alarm drive circuit generates and outputs the output voltage V13 having a predetermined value by varying a boosting ratio DC/DC converter 13. Accordingly, the DC/DC converter 13 requires a component for varying the boosting ratio, thereby complicating the circuit configuration.
Further, after the DC/DC converter 13 boosts the output voltage from the main battery 11 or the internal battery 12, the drive circuit 14 converts it into a high frequency voltage. For that reason, it is necessary to install both the DC/DC converter 13 and the drive circuit 14, thereby increasing the number of parts and the cost.
Accordingly, to simplify the circuit configuration, there has been disclosed a vehicle burglar alarm circuit which performs the conversion to a high frequency power by using a push-pull circuit and changes the boosting ratio of the transformer. The transformer of the vehicle burglar alarm circuit includes a tap switching unit for switching between a plurality of taps connected to a primary coil thereof. In the respective cases of being powered from the main battery and the internal battery, the boosting ratio can be varied by adjusting the coil ratio between the primary coil and the secondary coil connected to the sounding body as the taps of the first coil are switched by the tap switching unit. Accordingly, it is possible to sound the alarm having a satisfactory sound pressure level even when the sounding body is powered from the internal battery whose output voltage is lower than that of the main battery.
As such, the conventional vehicle burglar alarm circuit requires the transformer provided with the tap switching unit for varying the boosting ratio. Therefore, there have been demands for a vehicle burglar alarm circuit with a simpler circuit configuration.